This invention relates to an image pick-up tube, particularly, to apparatus, e.g., color television camera, in which the influence of the earth's magnetism is suppressed.
As is known well, it is impossible to neglect the influence of the earth's magnetism when attempting to improve the properties of a television camera. The influence of the earth's magnetism is particularly significant in a three tube color television camera. Specifically, the three tubes for picking up electric signals for red (R), green (G), and blue (B) light are not arranged in the same direction and, thus, differ from each other in the degree to which they are influenced by the earth's magnetism, rendering it difficult to perform the registration adjustment of the television camera.
In order to eliminate the influence of the earth's magnetism, it is customary to cover an image pick-up tube with a shield case as shown in FIG. 1. An image pick-up tube 11 is provided with a target 11a and an external connecting terminal 11b at the front and rear ends, respectively. A deflection coil 12 and a focusing coil 13 are provided around image pick-up tube 11. Further, image pick-up tube 11, deflection coil 12 and focusing coil 13 are housed in a cylindrical shield case 14. Shield caps 15 and 16 are mounted to the ends of shield case 14. The central portions of caps 15 and 16 define apertures 15a and 16a, respectively, so that target 11a and external connecting terminal 11b are exposed to the outside.
In a color television camera comprising three tubes, each constructed as shown in FIG. 1, the registration adjustment is performed as follows. First, an image pattern for registration adjustment is projected on a screen of a picture tube by, for example, allowing the television camera to face to the west with the output patterns of the red and green color image pick-up tubes made positive and negative images, respectively. Under this condition, both output patterns are made to coincide. Then, the television camera is allowed to face to the north, east and south successively so as to examine the degree of coincidence between the output image patterns of the red and green color image pick-up tubes.
The registration adjustment described above permits cincidence of patterns where the television camera is directed to the east because the influence of the earth's magnetism is substantially the same in the east-west direction. But, the patterns deviate from each other when the television camera faces to the north or south. FIG. 2A shows the deviation between a negative pattern A formed by the red color image pick-up tube and a positive pattern B formed by the green color image pick-up tube, where the television camera faces to the north. FIG. 2B covers the case where the television camera faces to the south. The deviation is caused by the relative displacement of the two output patterns in both rotary and linear directions. Therefore, the degree of deviation is greater in the peripheral portion of the screen, though the deviation in the central portion of the screen is not negligible.
For recently developed television cameras, the scanning lines are smaller in width to improve the picture image resolution. This leads to a more demanding registration requirement.
FIGS. 3 and 4 illustrate the cause of the pattern deviation discussed above. As shown in FIG. 3, the magnetic flux, denoted by arrows of solid lines, generated mainly by focusing coil 13 runs through shield case 14 and shield caps 15 and 16. If the television camera faces to the north, the magnetic flux of the earth's magnetism is thought to run also through shield case 14 and shield caps 15 and 16 as shown by arrows with broken lines. FIG. 4 illustrates a magnification of portion C of FIG. 3. It is seen that both shield case 14 and shield cap 15 have rough surfaces and, thus, fail to make contact over the entire overlapping area. Strictly speaking, shield case 14 and shield cap 15 make point-to-point contact. In other words, the magnetic path in portion D is so narrow that magnetic saturation is thought to occur in portion D. Under this condition, the portion magnetically saturated is expanded or shrunk depending on the direction of the earth's magnetism, so that the shielding effect of shield case 14 and shield caps 15 and 16 varies widely depending on the direction of the television camera. The variation of the shielding effect not only leads to variation in the degree of influence of the earth's magnetism around target 11a but also influences the distribution of the focusing magnetic field around target 11a.
To be brief, the pattern deviation is influenced by not only the earth's magnetism but also the variation in the distribution of the focusing magnetic field defining the electron beam passageway. The variation of the focusing magnetic field distribution is very small in general, but generates marked deviations in the image pattern. Specifically, the distance of the electron beam passageway from the axis of the image pick-up tube is the greatest around target 11a. In addition, the running speed of the electron beam is close to zero around target 11a because of the influence of the deceleration electric field. It follows that the directivity is so weak around target 11a that even a slight variation of the magnetic field gives marked influences to the electron beam, leading to the occurrence of pattern deviation. The variation of the focusing magnetic field is a serious defect in a television camera, particularly, in a broadcasting television camera wherein high precision is a necessity.